Control, or switch mats, which are typically used in such applications as actuating automatic pedestrian doors and providing safety areas around potentially dangerous machinery, function as simple electrical switches that operate under the weight of an object or person. The most common variety consists of a pair of flat parallel steel plates separated by an array of small, compressible, electrically insulating spacers, sealed within a tough vinyl jacket. Electrical leads are attached to the two plates. When an object or person of sufficient weight stands on the mat, the steel plates are forced into physical, and electrical, contact, thereby causing the electrical resistance between the mat leads to drop to a low value indicative of the presence of the person or object on the mat. When the object or person is removed from the mat, the contact between the steel plates is broken and the electrical resistance between the mat leads assumes an essentially infinite value indicative of the absence of sufficient weight, and, hence, the person or object, on the mat.
Control mats afford a number of distinct advantages over other methods of presence detection, such as microwave and infrared (IR) beam sensors. Unlike beam sensors, the detection area of a control mat is, generally, rectangular so that, in a pedestrian door application, control mats can provide detection coverage right up to the threshold and side-to-side to the door jambs. The detection area cannot be displaced by the sensor, in this case the mat, accidentally or intentionally being struck or tampered with, as can a beam sensor. Control mats are insensitive to variations of line voltage and, if proper interference suppression measures are employed, cannot be interfered with by nearby RF (radio frequency) transmitters or IR radiation from sunlight. Beam sensors, on the other hand, cannot be made completely immune to interference from RF and IR radiation because they use these media as their primary input. Also, control mats respond strictly to the weight of the person or object standing on them and they do not depend for detection upon any other special properties or characteristics, such as the state of motion or the microwave or IR reflectivity of the detected person or object, as do beam sensors.